The present invention relates to spinal rods and plates used in the stabilization of the human spinal column, and more specifically, to rods and plates that have variable shapes and mechanical properties over their lengths.
Spinal stabilization generally refers to fixation of the spinal column for the purpose of allowing fractured, compressed, or otherwise injured vertebra to heal, or for correction of malformed spinal curvatures. One method of spinal stabilization is to attach rigid rods or plates posteriorly and bilaterally to the spinal column to maintain vertebrae in a desired spatial relationship, or bring vertebrae into a desired curvature. Rods extend over at least two vertebra, and commonly over multiple vertebra, and are affixed to the spinal column using various means such as screws, wires, clamps, or combinations thereof. Plates also typically extend over at least two vertebra, but are more commonly affixed to the spine using screws.
Commonly, spinal rods are cylindrical, and have one consistent cross-section throughout their lengths. Absent a variation in materials within a rod, the size of the cross-section determines the strength of the rod.
However, in spinal surgery, a rod having a consistent cross-section, and therefore consistent mechanical properties throughout its length, is not always desirable. Generally, the loading differs at various spinal segments along the spine. Thus, a target location to be fixed may require application of different forces to improve the overall healing process. For example, it may be desirable for a lumbar segment of the rod to be more rigid than a thoracic segment.
Additionally, because of the change in the amount of space between the spinal column and the dermal tissue of a human back, from more space in the lumbar region to less space at the cervical region, the selection of the desired cross-sectional size of the rod is, in part, controlled by the amount of space available for its implantation. Thus, if a spinal rod that spans several cervical and thoracic vertebrae is desired to be of a certain cross-sectional size and strength for the thoracic region, and if that size is too large to implant into the cervical region, a smaller sized rod would have to be used. This would result in having an undesirably weaker rod fixing vertebrae that would benefit more from a stronger, larger rod. This holds true for spinal plates as well.
Thus, there remains a need for rod and plate designs with improved variability and control of mechanical properties, as well as rods and plates that allow a surgeon to select the different mechanical properties that are desired along the rod or plate, and where those mechanical properties should be located. There also remains a need for implantation procedures that facilitate pre-operative or interoperative planning to provide rigidity and flexibility at desired locations.